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The present invention relates generally to characterization of high-frequency electronic devices and more particularly to simulation and measurement of electronic devices using ground-signal-ground probes.
Ground-signal-ground (xe2x80x9cGSGxe2x80x9d) probes are used to make radio-frequency (xe2x80x9cRFxe2x80x9d) and microwave (xe2x80x9cMWxe2x80x9d) measurements of electronic devices. A GSG probe has a signal probe contact (xe2x80x9cfingerxe2x80x9d) between two ground probe fingers and can be used to measure electronic devices on wafer before they are separated for packaging. The GSG probe is brought in contact with the corresponding GSG pads on the electronic device and couples the device to a test instrument, such as a network analyzer, through a test cable.
Non-idealities in the test system (network analyzer, cables, fixture, etc.) introduce errors in the measured S-parameters that characterize the device under test (xe2x80x9cDUTxe2x80x9d). Some of these errors can be removed by calibrating the test system with calibration standards that provide equal (xe2x80x9cbalancedxe2x80x9d) currents through their ground paths. These errors are typically non-idealities in the signal path of the test system (xe2x80x9csignal path errorsxe2x80x9d). The errors in the test system will remain corrected as long as the current through the first ground finger of the GSG probe is assumed to be equal to the current through the second ground finger of the GSG probe when measuring an electronic device.
Unfortunately, some electronic devices do not produce balanced ground currents. If the ground currents of the measured electronic device are not equal, measurement errors, which are caused by non-idealities in the ground paths of the test system (xe2x80x9cground paths errorsxe2x80x9d), cannot be removed after calibration, resulting in an inaccurate measurement of the electronic device.
An interface circuit model of a GSG probe (hereinafter xe2x80x9cGSG interface modelxe2x80x9d) accounts for ground paths errors of a GSG probe that are not calibrated out using conventional calibration techniques. This GSG interface model can be thought of as existing at the interface between the calibrated GSG probe and a device under test (xe2x80x9cDUTxe2x80x9d), and can account for the measurement errors caused by unbalanced ground currents. In one embodiment, the GSG interface model includes a through path between a signal node and a signal source, a first inductance between a first ground node and a common ground, a second inductance between a second ground node and the common ground, and a mutual inductance between the first inductance and the second inductance.
The GSG interface model can be used to characterize electronic circuits having unbalanced ground currents. In one embodiment, a circuit model of an electronic device and a GSG interface model are entered into a simulator. The circuit model of the electronic device includes at least one signal port with associated first and second ground nodes (pads). The GSG interface model includes a first inductance between a common ground and the first ground node, a second inductance between the common ground and the second ground node, and a mutual inductance between the first self-inductance and the second self-inductance. A simulation of the electronic circuit and GSG probe is run to obtain a simulated characteristic of the electronic circuit and GSG probe. The simulated characteristic can be compared to the characteristic of the electronic device measured with the GSG probe, such as an on-wafer measurement of the electronic device.
In some embodiments, the value of the first inductance is equal to the value of the second inductance, and the value of the mutual inductance is the negative of the first inductance. In a particular embodiment, the first inductance and second inductance is Lcorr=(Lgxe2x88x92M2)/2, and the value of the mutual inductance is Mcorr=xe2x88x92(Lgxe2x88x92M2)/2, where Lg is the self-inductance of a ground finger of the GSG probe and M2 is the mutual inductance between the first ground finger and the second ground finger of the GSG probe.